A mobile device is generally provided with an internal battery which may be detachably installed into the mobile device and provide power to the mobile device for a variety of communication services. Although the internal battery is easy to carry, it does have some constraints in terms of a battery life and battery capacity. In particular, the battery life of the internal battery would not last too long when users play games or video files on the mobile devices, probably a few hours. Due to mechanical characteristic of the mobile device, it is impossible for the internal battery to possess relatively great battery capacity.